Potato mop-top virus (PMTV) - Fact Sheet

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Background

Potato mop-top virus (PMTV) causes an economically important disease of potato. Serious yield and quality reductions can occur in some cultivars. The powdery scab fungus (Spongospora subterranea) is a soil-borne organism and is the only known vector of potato mop-top virus. The distribution of S. subterranea in Canada is sporadic, but is more common in coastal areas where the cooler weather and wetter soils favour its development.

Distribution

• Asia
• Northern Europe
• North America
• South America

Biology

PMTV survives in the soil within dormant resting spores of its fungal vector S. subterranea . These infected viable resting spores can persist for up to 18 years in the soil.

High levels of soil moisture and cool soil temperatures (12-20°C) stimulate resting spores of the fungus to germinate and release virus-carrying zoospores. Zoospores can move only very short distances in soil and require free water for movement to occur. Zoospores introduce the virus into the potato plant when they infect the roots, stolons and/or young tubers. Systemic movement of the virus within the plant is generally slow and erratic. The critical period for infection and development of powdery scab on tubers is early in the growth cycle, at stolon formation and tuber set, a period that lasts about 3-4 weeks. Tubers which have matured beyond this period are resistant to infection by zoospores. Little or no spread occurs in areas where soil temperatures are above 20°C, or where moisture is lacking. Since roots can be infected by S. subterranea , even cultivars noted for some resistance to powdery scab may still be infected with PMTV.

When PMTV-infected tubers are planted as seed, the virus is passed on as a secondary infection to only limited numbers of progeny tubers (30 - 50%). Therefore, spread via the obligate vector, powdery scab, is the most important means of transmission.

Symptoms On Potato

Shoots and foliage:

• Symptoms may occur on the foliage of plants produced from infected tubers, and on stems, although not all stems produced will show symptoms. Foliage symptoms develop best at temperatures between 5 and 15°C, particularly during the early stages of plant growth. The most common symptom is the development of aucuba patterns on the stems which consist of bright yellow blotches and ring or line patterns on lower or middle leaves. Less commonly, a second type of symptom may be observed, consisting of pale, V-shaped, chlorotic chevrons, usually on the leaflets of young upper leaves, and ultimately resulting in a distinct mosaic in the upper leaves. A third type of symptom consists of extreme shortening of internodes accompanied by crowding or bunching of foliage, described as a "mop-top". Some of the smaller leaves may have wavy or rolled margins and the overall effect is a dwarfed and bunched growth habit.

Tubers:

• Although tubers are sometimes free of symptoms, in other cases raised, superficial concentric rings of 1 - 5 cm in diameter are present on the surface, radiating out from the point where the fungal vector first introduced the virus. Tubers may also develop brown necrotic lines, arcs and rings in the flesh, centred on the stolon attachment site. These symptoms are known as spraing. Spraing symptoms may not be present in tubers at the time of harvesting but can develop in storage, particularly if the storage temperature fluctuates. In other instances, depending on prevailing environmental conditions prior to harvest, spraing symptoms may occur at high levels at the time of harvest and increase very slowly during storage.

  Visual inspection of plants or tubers may be inconclusive for PMTV because the symptoms are similar to responses caused by other viruses or physiological conditions. In addition, foliar and tuber symptoms may not be present. The most reliable methods for accurately determining the presence of PMTV include: isolation of the virus using soil and bait plant methods, and detection through the use of indicator plants

Text: Plant Health Survey Unit & Plant Health Risk Assessment Unit

Photo Credits: Scottish Agricultural Science Agency (SASA) © Crown Copyright